1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, which varies the cam phase, i.e. the phase angle of at least one of an intake cam and an exhaust cam for opening and closing an intake valve and an exhaust valve, respectively, relative to a crankshaft of the engine, to thereby control valve timing of corresponding one(s) of the intake valve and the exhaust valve.
2. Description of the Prior Art
Conventionally, a valve timing control system of the above-mentioned kind was proposed in Japanese Laid-Open Patent Publication (Kokai) No. 7-269380. This valve timing control system includes a solenoid control valve, and a cam phase change mechanism to which oil pressure is supplied via the solenoid control valve. In this valve timing control system, an operation amount is output to the solenoid control valve, whereby the oil pressure is supplied to the cam phase change mechanism via the solenoid control valve. The cam phase change mechanism includes two hydraulic chambers i.e. an advance chamber and a retard chamber. The oil pressure from the solenoid control valve is selectively supplied to one of the two hydraulic chambers, whereby the phase angle (hereinafter simply referred to as xe2x80x9cthe cam phasexe2x80x9d) of an intake cam relative to a crankshaft is advanced or retarded to thereby change the valve timing (opening/closing timing) of an intake valve.
Further, desired valve timing is calculated based on engine rotational speed and the amount of intake air, and the solenoid control valve is feedback-controlled such that detected valve timing becomes coincident with the desired valve timing. During the feedback control, a hold operation amount (operation amount at which the cam phase change mechanism neither advances nor retards the cam phase) is learned to enhance the accuracy of feedback control, and a leaned value of the hold operation amount is used to calculate the operation amount. The learned value is calculated by adding an amount of deviation of the operation amount to the current operation amount when the rate or speed of change in the valve timing is within a predetermined small range, and at the same time the amount of change in the operation amount is within a predetermined small range, that is, when the valve timing undergoes very small changes. The amount of deviation of the operation amount is read from a map set in advance such that values of the amount of deviation are indexed by respective values of the rate of change in the valve timing.
According to the above conventional valve timing control system, so long as the condition that the valve timing undergoes very small changes is satisfied, the calculation of the learned value is carried out irrespective of whether or not the cam phase change mechanism is in normal operation, or irrespective of whether or not the desired valve timing is being changed. Hence, it is sometimes impossible to obtain an appropriate learned value, resulting in the degraded accuracy of the feedback control. Further, although the calculation of the learned value is carried out when the valve timing undergoes very small changes, such a state of the valve timing is often terminated in a short time during actual operation of the internal combustion engine. This sometimes causes the learning process to be terminated before obtaining a sufficient number of samplings to calculate the appropriate learned value. In such a case, the use of the learned value results in the degraded accuracy of the feedback control.
It is an object of the invention to provide a valve timing control system for an internal combustion engine, which is capable of enhancing the accuracy of valve timing control.
To attain the above object, the present invention provides a valve timing control system for an internal combustion engine having a crankshaft, an intake valve, an exhaust valve, an intake cam for opening and closing the intake valve, and an exhaust cam for opening and closing the exhaust valve, the valve timing control system controlling valve timing of at least one of the intake valve and the exhaust valve by changing a cam phase which is a phase of at least one of the intake cam and the exhaust cam, relative to the crankshaft.
The valve timing control system according to the invention is characterized by comprising:
actual cam phase-detecting means for detecting the cam phase as an actual cam phase:
a cam phase change mechanism for changing the cam phase:
operating condition-detecting means for detecting operating conditions of the engine;
fuel cut-off determination means for determining whether or not fuel cut-off is being carried out for cutting off supply of fuel to the engine;
desired cam phase-setting means for setting a desired cam phase according to the detected operating conditions of the engine, and fixing the desired cam phase at a constant value during the fuel cut-off;
cam phase control means for controlling the cam phase change mechanism such that the cam phase change mechanism causes the cam phase to become equal to the desired cam phase, and holds the cam phase at and after a predetermined timing during the fuel cut-off; and
actual cam phase deviation calculation means for calculating an amount of deviation of the actual cam phase based on a plurality of values of the actual cam phase detected at and after the predetermined timing during the fuel cut-off, and the constant value of the desired cam phase.
According to this valve timing control system, it is determined whether or not fuel cut-off is being carried out, and during the fuel cut-off, the desired cam phase is fixed at a constant value. Further, the cam phase change mechanism is controlled such that it causes the cam phase to become equal to the desired cam phase, and holds the cam phase at and after a predetermined timing during the fuel cut-off. The amount of deviation of the actual cam phase is calculated based on a plurality of values of the actual cam phase detected at and after the predetermined timing during the fuel cut-off and the constant value of the desired cam phase. Since fuel is not burned during the fuel cut-off, there is no need to change the desired cam phase, and hence the desired cam phase can be fixed to the constant value, as described above. Further, the fuel cut-off is not terminated immediately but often continues over a certain time period. Therefore, since the amount of deviation of (a detected value of) the actual cam phase from a correct value to be detected is calculated in a state in which the desired cam phase is fixed, and the cam phase is held after the predetermined timing, it is calculated not only based on the actual cam phase which has sufficiently converged on the desired cam phase but also when the converged state of the actual cam phase continues over a certain time period, differently from the conventional valve timing control system in which learning is carried out on condition that the valve timing undergoes very small changes, irrespective of whether or not the desired cam phase is being changed. This makes it possible to calculate a learned value of the amount of deviation more accurately reflecting an actual amount of deviation of the actual cam phase, whereby the amount of deviation of the actual cam phase can be calculated with higher accuracy. This makes it possible to correct the actual cam phase during the valve timing control by using a thus accurately calculated and hence a reliable learned value of the amount of deviation of the actual cam phase, and thereby enhance the accuracy of the valve timing control.
Preferably, the valve timing control system further includes actual cam phase-integrating means for integrating an amount of change in the actual cam phase before the fuel cut-off to obtain an integrated value, and calculation-permitting means for permitting the actual cam phase deviation calculation means to calculate the amount of deviation of the actual cam phase when the integrated value is equal to or larger than a predetermined value.
According to this preferred embodiment, the calculation of the amount of deviation is permitted on condition that the integrated value of the amount of change in the actual cam phase before the fuel cut-off is equal to or larger than the predetermined value. Generally, when the cam phase change mechanism is in operation without being inoperatively fixed, the integrated value of the amount of change in the actual cam phase becomes larger with the lapse of operation time of the mechanism. Therefore, by permitting the calculation of the amount of deviation of the actual cam phase on condition that the integrated value is equal to or larger than the predetermined value, it is possible to sample only data of the actual cam phase when the cam phase change mechanism is in operation while eliminating data of the same when the mechanism is inoperatively fixed. This makes it possible to fully enhance the reliability of the learned value of the amount of deviation of the actual cam phase, and hence further increase the accuracy of the valve timing control.
Preferably, the valve timing control system further includes follow-up delay determination means for determining based on a difference between the desired cam phase and the actual cam phase whether or not there occurs a follow-up delay of the actual cam phase with respect to the desired cam phase, and second calculation-permitting means for permitting the actual cam phase deviation calculation means to calculate the amount of deviation of the actual cam phase when it is determined by the follow-up delay determination means that there does not occurs the follow-up delay.
According to this preferred embodiment, the calculation of the amount of deviation of the actual cam phase is permitted when it is determined based on the difference between the desired cam phase and the actual cam phase that there does not occur a follow-up delay of the latter with respect to the former. In general, when the cam phase change mechanism is in normal operation, there does not occur the follow-up delay or the like, so that the difference between the desired cam phase and the actual cam phase is small, and a state, for instance, in which the difference is excessively large cannot continue for a long time period. Hence, it is possible to determine, based on the difference, whether or not there occurs the follow-up delay, i.e. delay of the actual cam phase in following up the desired cam phase. Therefore, by permitting the calculation of the amount of deviation of the actual cam phase when there occurs no follow-up delay, as described above, it is possible to sample data of the actual cam phase when the cam phase change mechanism is in normal operation, in other words, when the actual cam phase has converged on the desired cam phase. This makes it possible to further enhance the reliability of the learned value of the amount of deviation of the actual cam phase, and hence further increase the accuracy of the valve timing control.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.